Two members of the New Black Panthers in St. Louis planned to use the chaos in the post-Darren Wilson grand jury announcement to bomb the iconic Gateway Arch and kill prosecutor Bob McCulloch and Ferguson top cop Tom Jackson, local media reports.

But the plot went deeper than that. The two bought what they thought was a pipe bomb in a federal sting, and were thwarted from buying more because they needed to wait to get more cash on one man's girlfriend's Electronic Benefit Transfer card, sources told the St. Louis Post-Dispatch.

EBT cards are used to distribute welfare funds to low-income households.

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The men allegedly tried to buy two Hi-Point .45-caliber pistols for someone else sometime between Nov. 1 and Nov. 13 at a Hazelwood, Mo. Cabela's, the federal indictment alleges. Both are charged with aiding and abetting the making of a false written statement made in connections with a firearms purpose.

The shooting death of 18-year-old Michael Brown on Aug. 9 set of weeks of protests culminating in riots after Darren Wilson was not charged Monday.

The suspects were arrested four days before McCulloch, the St. Louis county prosecuting attorney, announced that a grand jury declined to indict Wilson in the fatal August shooting of unarmed teen Michael Brown.

Both McCulloch and Jackson, the Ferguson police chief, were criticized for their respective roles in the investigation and handling of the police-involved shooting. It's unclear how Davis, who was identified by police as the leader in the plot, and Baldwin planned to assassinate the men.

Sources tell the Post-Dispatch that each official has had a police presence outside their homes as the decision neared.

Jackson "was warned about a plot" by the feds but was "not given a lot of detail," KSDK-TV reported.

Authorities recorded interactions with the men on hidden camera.

Davis and Baldwin hoped to bring at least one pipe bomb up the Gateway Arch and hide it on the observation deck, though it's unclear how because the site has airport-style screening, according to the newspaper.

The forthcoming issue of the New Yorker magazine depicts the arch, black on one side and white on the other with a gap in the middle, to show the racial tension in St. Louis surrounding the Brown shooting.

The New Black Panthers blasted the charges as "trumped up and baseless," writing in a statement that the allegations are "totally unfounded."

Brandon Orlando Baldwin is also named in the indictment, and could face more charges. (Facebook)

"We do not condone aggression in deed or action!" the statement reads. "The men accused are innocent until proven guilty, and no matter what must be handled as such, until the facts are presented."